I first want to preface this by saying I am not mechanically ignorant, I understand the basics of how derailleurs work, but I need the help of the big brains of weight weenies.

My issue is this.My SRAM red black rear derailleur shifts beautifully on a SRAM xg1090 (11-28) cassette except for the jump from 12T->11T, my previous 11-25 cassette had the same issue. The cassette is on a meilenstein 10 speed rear wheel. The chain (KMCx10sl) will make the jump but it often pauses before doing so. When in the 11T it makes a slight rattling sound like it is not perfectly seated on the sprockets but will never actually skip. The skipping can be manually stopped by pulling the derailleur body, slightly, away from the rear wheel. Now, I know what you are thinking - too much cable tension - I have tried unwinding the tension and it still has made no difference. The other thing you are thinking is the limit screws, once again I have looked at this, the derailleur does not actually touch the outer limit screw when in the 11T. I have adjusted the B screw as well thinking it might be lack of chain wrap causing the lack of smoothness but this too is not an issue.

I am currently running Gore ride-on cables with KCNC Ti cable, but the same issue existed with the gore standard steel cable too. The cable is clamped correctly to the derailleur body (I have another identical SRAM derailleur on another bike that is functioning perfectly to comapre against). I have checked adequate lubing of the bottom bracket cable guide.

I have previously removed by inner and outer cage of the rear derailleur for sanding, this freed the parrellogram spring, but i believe it was reinstalled correctly and the only jump that isn't smooth is the 11-12T.

Also, make sure your loop is long enough going to the rear derailleur. SRAM rear derailleurs tend to work better with a slightly longer loop of cable.

you can test if its the cable or not buy disconnecting the cable from the derailleur entirely, and see if it shifts all the way down. If it doesn't then you've got to align the derailleur hanger. If that doesn't work, you can stick a washer between the hanger and the derailleur.

I'm not an expert, but I spent 3 days recently tuning my Red RD and I was having the same issue as you on my 11t and 12t. Here is what helped me:

- Run the cables on the back of the bars. This creates less friction, and seemed to help.- With the cable disconnected, adjust the L limit screw so that the pulley teeth are lined up with the outer edge of the teeth on the smallest cog. This is a big one for my RD. It won't shift into the smallest cog if they are all center aligned.- Screw in the cable adjuster all the way in, and back it out a few turns, then connect the cable.- Shift from 11 to 12 to 11 and adjust the tension of the cable with the adjuster until it shifts smoothly.

You also mentioned that the derailleur doesn't touch the limit screw when in the smallest cog. From my understanding, it should touch, and if it doesn't then the tension is too high.

I also have a problem with my relatively new SRAM Red RD smallest to smallest+1 (12-13 in my case). Except for the transition 12-13 all gears shift smoothly, but from 12 to 13 it jumps to 14 before immediately falling back to 13. I haven't gotten around to figuring it out, but I am thinking it might be too high cable tension. Will work on it this weekend.

The shop has checked my hanger and say it is okay. I don't have an alignment tool.The length of RD cable is cut to the recommended length and looks the same length as my other SRAM bike, the cable is routed on the outside of the handlebars.I can manually hold it in the 11T and have it running smoothly so there is movement capacity in the derailleur to get there.

Thanks everyone for help, I have finally worked out what it is. There is a small screw on the lower parellogram arm that secures the inner derailleur cage. When I reattached the inner cage after dissambling it i mustn't have wound the screw all the way in. It was perhaps 0.5mm out. When the lower parellogram arm moved in towards the derailleur body it caught on the screw and stopped its movement. Now with the screw all the way in it is shifting like my other derailleur. I am amazed something so small made such a difference and that it has taken 7000k's to detect the cause.

NiFTY - you may want to pull that retaining pin and check it, as well as the cage pivot spindle, for wear... I had an issue with a 2010 SRAM Red short cage rear derailleur where I noticed the cage would ramp "in and out" from the body (which was difficult to notice with black on black parts) when the cage was rotated (as if taking up chain slack). When I removed the cage from the body, the pin had a rough spot where it would contact the cage pivot spindle. There is a machined flange near the end of the spindle that rides behind the pin (when it is installed) and holds things together. One side of this flange was almost eroded through - this created a "ramp" on the flange which caused the "in and out" movement of the cage. Shifting up and down the cassette, the cage was constantly changing position relative to the rest of the derailleur causing similar "adjustment" headaches. The cage spindle will eventually fail, most likely sending the cage into your wheel. I did find a couple other posts on the interweb showing failed pieces. SRAM wouldn't touch mine as "it was outside of their 2 year warranty", which I sort of get. I was really buggered that my LBS wouldn't cover it with their "Lifetime" parts warranty...

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